Preheat oven to 350°F.
In a small saucepan, heat the olive oil over medium-high heat. Add the onions and cook, stirring, for 1 minute. Add the pear, sugar, juice and balsamic; stir as it comes to a boil.
Reduce heat to medium and cook, stirring occasionally, until liquid is reduced to just a couple tablespoons and it has a thick, syrupy consistency, about 12 to 15 minutes. Transfer to a bowl to cool a bit while you warm the Brie.
Place Brie round on an oven-safe serving dish or tray and place in the oven to warm for about 8 minutes – the goal is not to melt the cheese, it’s to simply soften it a bit so it’s easier to slice and spread.
Meanwhile, stir the pecans into the caramelized sauce.
Once Brie is warmed, remove from oven and spoon the sauce over the Brie round. Serve immediately.